With a view to effectively re-using a heat exhausted gas, heat exchangers have been used in a variety of fields such as heat exchange plates of a gas-fired water heater or Eco-cute body. In the gas-fired water heater, a mixed liquid containing nitric acid, sulfuric acid, and chlorine is generated as a result of condensation of the components of an exhaust heat gas and there is therefore a fear of deterioration in corrosion resistance occurring, after liquid/liquid heat exchange, on the heat exchange plate due to chlorine contained at a high concentration. In addition, since Ni-based filler metal or Cu-based filler metal is employed for joining of parts, it is necessary to avoid deterioration in ductility/toughness of them due to coarsening of the structure upon brazing. For members which are required to have corrosion resistance and brazing ability, copper or copper alloy has conventionally be used. Copper is inferior in strength so that the member must be thickened in order to heighten its strength, which leads to a problem of cost increase. Accordingly, austenitic stainless steels such as SUS304 and SUS316 have so far been used as a substitute for copper in a steel member for heat exchangers.
A member to be brazed is required to have the following properties:    (1) Various good brazing abilities such as brazing ability with Ni-based brazing metal, brazing ability with Cu-based brazing metal and torch brazing ability with inexpensive brass-based filler metal or the like.
Further, when a member to be brazed is a metal member of heat exchanger (refrigerant pipe or water pipe) etc, it is further required to have the following property:    (2) Good corrosion resistance under an environment of nitric acid or sulfuric acid in a condensate derived from a combustion gas or good crevice corrosion resistance under an aqueous environment having a high chlorine concentration.
As a material having such properties and a good brazing ability, a ferritic stainless steel is investigated in Japanese Patent No. 2642056.
[Patent Document 1] Japanese Patent No. 2642056